The intelligence came from an exotic variety of sources: there was the so-called Laptop of Death; there was the Iranian commander who mysteriously disappeared in Turkey. Also in the mix was video footage of a nuclear plant in central Iran and intercepts of Iranian telephone calls by the British listening station GCHQ.

But pivotal to the US investigation into Iran's suspect nuclear weapons programme was the work of a little-known intelligence specialist, Thomas Fingar. He was the principal author of an intelligence report published on Monday that concluded Iran, contrary to previous US claims, had halted its covert programme four years ago and had not restarted it. Almost single-handedly he has stopped - or, at the very least, postponed - any US military action against Iran.

His report marks a decisive moment in the battle between American neoconservatives and Washington's foreign policy and intelligence professionals - between ideologues and pragmatists. It provided an unexpected victory for those opposed to the neocon plans for a military strike.

The report, the National Intelligence Estimate (NIE), which represents the consensus of the 16 US intelligence agencies, gave President George Bush one of his most difficult weeks since taking office in January 2001.

Fingar's findings were met in many Washington offices occupied by foreign policy and intelligence professionals not only with relief but with rejoicing. They had lost out in the run-up to the war in Iraq in 2003, but they are winning this one.

A backlash is under way; with the neocons being joined by even moderate foreign policy specialists who claim the report seriously underestimates the threat posed by Iran. Senate Republicans are planning to call next week for a congressional commission to investigate the report.

Senator John Ensign, a Republican, said: "Iran is one of the greatest threats in the world today. Getting the intelligence right is absolutely critical."

Flynt Leverett, a former CIA analyst and former National Security Council adviser in the Bush administration, was among those celebrating this week, and praised Fingar and his colleagues. "We seem to have lucked out and have individuals who resist back-channel politics and tell it how it is," he said. "That is what the CIA and other agencies are supposed to do."

He continued that Fingar and one of his co-authors, Vann Van Diepen, national intelligence officer for weapons of mass destruction, had opposed the war in Iraq. "They both felt the intelligence was misused in the run-up to the Iraq war. The conservatives are now attacking them, saying they are taking their revenge," Leverett said. "It is not mutiny for intelligence officers to state their honest views."

Fingar, Van Diepen and Kenneth Brill, a former US ambassador to the International Atomic Energy Agency (IAEA), were able to put out what they regard as an objective assessment because those occupying senior roles in the Bush administration had changed. Paul Wolfowitz, John Bolton, Douglas Feith and Donald Rumsfeld have given way to those who oppose war with Iran, including Robert Gates, the defence secretary and former CIA director, and the secretary of state, Condoleezza Rice.

Only the vice-president, Dick Cheney, remains to advocate military strikes against Iran. Wolfowitz, out of work since resigning from the World Bank earlier this year, has been invited back into the administration by Rice as an adviser on WMD, but that is an act of pity for an old mentor, not a shift in power to the neocons.

Joseph Cirincione, author of Bomb Scare: The History and Future of Nuclear Weapons, also welcomed the report, saying: "What is happening is that foreign policy has swung back to the grown-ups. We are watching the collapse of the Bush doctrine in real time. The neoconservatives are howling because they know their influence is waning."

The report is a disaster for Bush's Iranian policy. Although he still refuses to take the military option off the table, it is harder to give the order to go to war. It also makes it harder for the US to persuade Russia and China to back tougher economic sanctions against Iran.

Bush and Cheney might have tried to block publication but feared it would leak, leading to damaging charges of cover-up and the manipulation of intelligence. "It was not likely to stay classified for long, anyway," Cheney told Politico, the Washington daily devoted to politics.

The "howling" of the neocons that Cirincione spoke about began within hours of the report's publication. Bolton, who remains close to Cheney, appeared on CNN complaining about the authors without naming them. In the comment section of the Washington Post on Thursday he wrote: "Many involved in drafting and approving the NIE were not intelligence professionals but refugees from the state department." He accused the officials, who he said had held benign views of Iran's nuclear intentions five or six years ago, of presenting these same policy biases as "intelligence judgements".

The Wall Street Journal, the editorial pages of which have long been aligned with the neocon agenda, went straight on to the attack within a day of the report's publication, expressing doubt in the officials and their conclusions. It quoted an intelligence source describing Fingar, Van Diepen and Brill as having reputations as "hyper-partisan anti-Bush officials".

Bush has said repeatedly that the US will not allow Iran to secure a nuclear weapons capability. Air strikes were becoming an increasingly likely option, even though opposed by the US state department, the Pentagon and the intelligence agencies. As of last spring, American deployments in the Gulf had been completed, ready should the order be given.

A European official close to the discussions, who is copied in to key memos relating to Iran, spoke in the summer as if an attack was a given. He said that the war was containable only as long as the Iranians did not strike back.

Vincent Cannistraro, a former CIA head of counter-terrorism, said Bush had been building up the anti-Iran rhetoric to justify the military option: "There was a set of contingency plans updated over the last year and a half. The intent was air strikes to destroy the nuclear programme to the extent that it could be done. Is it possible to destroy 100% underground nuclear facilities? No, it is not. Could they set it back 10 years? Yes."

Iran's covert programme can be traced back to the mid-1980s when the country was at war with Iraq and fearful that the then Iraqi leader, Saddam Hussein, might secure a nuclear weapon. The programme involved design, ballistic delivery systems and uranium enrichment; the NIE concluded in 2005 that it was continuing. In July that year US intelligence officials showed IAEA officials an alleged stolen Iranian laptop with thousands of pages relating to nuclear weapons experiments. It was nicknamed the Laptop of Death - it is still not clear whether it was genuine

Fingar and his colleagues have gone back over the material and subjected it to a higher level of scrutiny. They took the same data but reached different conclusions. They also had some new material.

Cannistraro said everyone was pointing towards General Ali-Reza Asgari, a former deputy defence minister, who disappeared in Turkey in February. But he insisted Asgari had been a long-term agent run by the West who has since been debriefed and given a new identity.

"It is not a single source," said Cannistraro. "It is multiple: technical, documents, electronic."

Cheney, though his position is weakened by the NIE report, is due tomorrow to give a TV interview in which he will insist that the danger posed by Iran has not diminished. He told Politico the cause for concern was Iran's civilian development of highly enriched uranium, which would be relatively easy at a later date to switch to making a nuclear weapon.

Foreign policy pragmatists are pressing for the US to open direct talks with Iran. This is unlikely, but what this week has meant is that Bush has lost one of the pretexts for launching a new war in the 13 months he has left in office.